The present invention is a method and apparatus that provides a system for calibrating the ion current projected from an ionographic print head to achieve a uniform value. Uniformity of ion current under conditions of a known modulation voltage is important in producing high quality printed images, particularly those in which a large number of electrodes are driven to produce large grey areas. Non-uniform densities of ion current often result in unacceptable printed image density.
Ionographic printing is that type in which charged particles are created at a remote location from the point at which a charge latent image is formed on a dielectric surface of a conductive substrate receptor. A stream of ions, assisted in some known systems by a fluid stream, generally an air stream, passes through a channel in the print head in which is placed a linear array of modulation electrode fingers, one for each pixel in a line of the printed image. Ions are created in a chamber, for example, by electrical discharge from a corona wire, maintained at very high positive voltage. During the printing process, the stream of ions, or current, is projected toward the receptor, usually a drum, which is placed at a high, negative electrical potential so as to attract the ions to its dielectric surface. Those ions which are projected from the print head as a current are pulled to the dielectric, over-coated surface of the receptor where they form a charge image, ready for immediate Xerographic-type development into a printed image. Application of low, modulation voltages to the electrode fingers can deflect the ions and locally eliminate them from the current, producing varying densities of ions and thus varying density of the subsequently printed image.
In high-quality ionographic printing, particularly where grey-levels are used, unacceptable variations in the printed image density occur unless non-uniformities of ion current are corrected. Prior art has addressed the problem of controlling ion perturbation at the imaging surface caused by the effect of previously deposited ions on the path of subsequent ions directed toward the imaging surface. A U.S. Patent Application entitled "FEEDBACK SCHEME FOR IONOGRAPHIC CALIBRATION", filed Dec. 26, 1990 and assigned to the same assignee as the present application, discloses an array of calibration electrodes, each electrode associated with an opposing modulation electrode and coupled to the opposing modulation electrode through a feedback circuit which adjusts the ion current associated with the modulation electrode to a uniform value at a given value of modulation voltage. Such a device has sufficient spatial resolution to unambiguously measure the ion current associated with each individual modulation electrode. The calibrated modulation voltages for the print head are stored for later use during printing or used in real time depending on the speed of printing desired. As used herein, "calibrating" is the process of making the ion stream uniform for a desired value of modulation voltage. The referenced device would likely occupy space between the print head and the receptor. It requires a large number of individual feedback circuits, possibly as many as the number of modulation electrodes, depending on the design of the print head and associated multiplexing circuits for addressing the modulation electrodes.
The problems of eliminating mechanical scanning and of rapidly calibrating a large array of modulation electrodes in an ionographic head have been major ones for designers of ionographic printing devices. It would be a major technological advance to provide a simpler way to eliminate the problems of mechanical scanning a large array of modulation electrodes in an ionographic head, at a low cost, and particularly if the device could be easily mounted directly to the print head. The development of such a direct way to insure uniform ion flow in spite of variations in the construction of ionographic printing heads would fill a long felt need in electronic printing.